An intake/exhaust valve (hereinafter, referred to as “poppet valve”) used in an internal combustion engine of marines and the like has a basic configuration in which a valve head is formed integrally with one end of a valve stem in a longitudinal direction, and the valve head of the poppet valve is provided with a seat surface formed to be inclined in a direction where an outer diameter becomes larger toward the one end of the valve stem. The poppet valve performs a linear/reciprocal motion in a cylinder head in synchronization with timings of intake and exhaust of the internal combustion engine. In these timings, the seat surface of the poppet valve repeatedly collides with a metallic seat ring disposed in an opening of each of intake and exhaust ports, and therefore the stiffness strength of the seat surface of the poppet valve is required.
Patent Document 1 discloses the technique in which in an intermediate valve products before forging, a pressing protrusions is provided in a region serving as a seat surface of a poppet valve that will be finally obtained, and the seat surface of the poppet valve that will be finally obtained is subjected to work hardening by pressing the pressing protrusions at forging for plastic-deformation, thus improving the stiffness strength of the produced poppet valve.
Patent Document 1 is Japanese Patent Laid-Open No. 2001-123256 A.
FIG. 6 shows a method for forging a poppet valve 500 disclosed in Patent Document 1, wherein FIG. 6 (a) shows a shape of an intermediate valve products 500′ before forging, and FIG. 6 (b) shows a shape of an intermediate valve products 500 after forging.
In FIG. 6 (a), a shape (outer shape) of a seat surface in the intermediate valve products 500 will be finally obtained is illustrated in a broken line, and is illustrated to overlap the intermediate valve products 500′ before forging.
As illustrated FIG. 6(b), the intermediate valve products 500 after forging have a basic configuration in which a valve head 502 is formed integrally with one end of a valve stem 501 in a longitudinal direction. The valve stem 501 is a bar-shaped member linearly extending along a center axis X1 of the valve stem 501, and the valve head 502 formed in one end of the valve stem 501 includes a columnar margin portion 503 and a seat portion 504 having an outer diameter in a radial direction of the center axis X1 that is the smaller according to being more away from the margin portion 503. The seat portion 504 is provided with a seat surface N that is inclined at a predetermined angle θ with respect to the center axis X1, and in a sectional view, and the seat surface N is formed in a planar shape toward the center axis X1 from a boundary point 503a to the margin portion 503.
As illustrated in FIG. 6 (a), the intermediate valve products 500′ before producing the poppet valve 500 by forging are provided with a swollen portion 505′ swelling in a direction of being away from a margin portion 503′ in a region serving as the seat portion 504 after forging.
Here, for forging the shape of the poppet valve 500, the intermediate valve products 500′ are held in a state where the pressing protrusions 505′ is in contact with a pressing surface 520a of a die 520 as a forging die. In this state, a punch 540 arranged to face the die 520 is moved to the die 520 along the center axis X1, and thereby a shape of an intermediate valve head 502′ in the intermediate valve products 500′ is adjusted to a shape of the valve head 502 in the poppet valve 500 that will be finally obtained.